A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. However, deposition is not restricted to the surface of the substrate, and this can result, for example, in the clogging of gas nozzles and the clouding of chamber windows. In addition, particulates may be formed, which can fall on the substrate and cause a defect in the deposited thin film, or interfere with the mechanical operation of the deposition system. As a result of this, the inside surface of the process chamber is regularly cleaned to remove the unwanted deposition material from the chamber.
One method of cleaning the chamber is to supply molecular fluorine (F2) to react with the unwanted deposition material. Fluorine is either supplied at a high purity (at least 99% fluorine), or supplied diluted with nitrogen in the ratio of 20% F2 to 80% N2. In the cleaning process, the residence time of the fluorine in the process chamber is relatively short, and so only a small proportion of the F2 gas supplied to the chamber is consumed during the cleaning process. Consequently, the majority of the cleaning gases supplied to the chamber are exhausted from the chamber together with by-products from the cleaning process.
In order to remove fluorine from the gas stream exhaust from the process chamber before the gas stream is exhaust to the atmosphere, an abatement device such as a thermal processing unit or plasma abatement device is typically provided to convert the F2 into water-soluble hydrogen fluoride. The gas stream is subsequently conveyed to a wet scrubber, wherein the HF is taken into aqueous solution. The aqueous HF is then conveyed from the scrubber to an acid drain, or more commonly to a fluoride treatment facility, where a compound such as calcium hydroxide is typically used to neutralise the aqueous HF and precipitate from the aqueous HF a “cake” or “sludge” containing CaF2. Such fluoride treatment facilities tend to be expensive, and are often capacity limited. Furthermore, disposal of the CaF2 cake also tends to be expensive.